1. Field of the Invention
The present invention relates to media sensors, and, more particularly, to a method to correct for sensitivity variation of media sensors.
2. Description of the Related Art
A typical media sensor includes a single light source, such as a light emitting diode (LED), and a reflectance detector, such as a phototransistor. Also, typically, the reflectance detector is located on the same side of a media as the light source. During operation, the LED directs light at a predefined angle onto a material surface of the media, and the surface characteristics of the media are examined in terms of the amount of light reflected from the surface that is received by the photo detector. The presence of the media is detected based upon a predetermined amount of light reflected from the media to the reflectance detector.
Some sensors include a pair of reflectance detectors, one of the reflectance detectors being positioned to sense reflected diffuse light and a second detector positioned to sense reflected specular light. Such a sensor may be used, for example, to detect and discriminate between paper media and transparency media.
Media sensors that detect the type of media in an imaging device, such as an ink jet printer, optically measure the glossiness of the media using a media sensor similar to that described generally above. To measure the glossiness, a collimated beam of light is directed towards the media and a reflectance ratio (R) of the detected reflected specular light intensity and the detected diffusively scattered light intensity is calculated. The media sensor is initially calibrated by measuring a reflectance ratio (R0) on a known gloss media. A normalized reflectance ratio (Rn) is calculated using the formula: Rn=(R/R0). Normalized reflectance ratio Rn then is used to identify the media type of an unknown media by a comparison of normalized reflectance ratio Rn to a plurality of normalized reflectance ratio Rn ranges, each range being associated with a particular type of media. For example, if the media sensor is calibrated with a perfectly diffuse media, then the normalized reflectance ratio Rn ranges might be as in the following table.
Out of the two light signals forming the ratio (R), the specularly reflected light is highly directional and its measurement is critical. The sensitivity of a media sensor to media type depends on how well the incident light is collimated and how well the specular light is captured without too much diffuse light contribution.
Variations of the optical components forming the media sensor can cause a significant spread in the sensor sensitivity. Spread in sensor sensitivity means, for example, that two sensors calibrated to the same media would result in different Rn values on a high gloss media. For example, one sensor might read the Rn value of a high glossy photo paper as Rn=8.0 while another sensor reads it as Rn=9.0. If the spread gets too large, such as if one sensor might read a photo paper as Rn=7.0 and another sensor reads the same photo paper as Rn=11.0, then the generation of a reliable test table, like that of Table 1 above, becomes difficult to achieve.
What is needed in the art is a method to correct for sensitivity variation of media sensors.
The present invention provides a method to correct for sensitivity variation of media sensors. With the present invention, a media sensor is calibrated using at least two media of different glossiness.
The invention, in one form thereof, relates to a method of correcting for sensitivity variation of media sensors. The method includes the steps of determining a first reflectance ratio of a first calibration media having a first glossiness; determining a second reflectance ratio of a second calibration media having a second glossiness, the second glossiness being greater than the first glossiness; and determining a corrected normalized reflectance ratio of an unknown media based on the first reflectance ratio of the first calibration media and the second reflectance ratio of the second calibration media
In another form thereof, the present invention relates to a method of dynamically correcting for sensitivity variation of media sensors during a determination of a media type of an unknown media using a first media sensor. The method includes the steps of determining a first reflectance ratio R0 of a first calibration media having a first glossiness; providing a predetermined average normalized reflectance ratio RA of a plurality of normalized reflectance ratios R1, R2, . . . RX for a second calibration media having a second glossiness, the second glossiness being greater than the first glossiness, the plurality of normalized reflectance ratios R1, R2, . . . RX being generated using a plurality of media sensors; determining a second reflectance ratio R[HIGH] of the second calibration media; determining a reflectance ratio R of the unknown media; and determining a corrected normalized reflectance ratio of the unknown media based on the first reflectance ratio R0 of the first calibration media, the predetermined average normalized reflectance ratio RA of the second calibration media, the second reflectance ratio R[HIGH] of the second calibration media, and the reflectance ratio R of the unknown media.
In still another form thereof, the present invention relates to a method of determining a corrected normalized reflectance ratio of an unknown media having an unknown media type. The method includes the steps of providing a first media sensor for sensitivity correction, the first media sensor being used in determining a reflectance ratio R of the unknown media type; selecting a first calibration media having a first glossiness; selecting a second calibration media having a second glossiness, the second glossiness being greater than the first glossiness; determining a first reflectance ratio R0 of the first calibration media; determining a plurality of normalized reflectance ratios R1, R2, . . . RX of the second calibration media; calculating an average normalized reflectance ratio RA of the plurality of normalized reflectance ratios R1, R2, . . . RX; determining a second reflectance ratio R[HIGH] for the second calibration media; and determining a corrected normalized reflectance ratio of the unknown media based, in part, on the first reflectance ratio R0 of the first calibration media, the average normalized reflectance ratio RA of the second calibration media and the second reflectance ratio R[HIGH] for the second calibration media.
An advantage of the present invention is that the present invention permits the use of media sensors having a wider tolerance range with no decrease in media sensing accuracy.